Being able to use a video as a post's featured media was previously discussed in #43117 and #41127. It was closed as wontfix.
I don't think the "featured media" is going to be the right solution here, especially blocks. I would think that a site wanting this feature should rather do something like designate a Group block as the featured container (e.g. as the first block in the content), within which any block can be added. This could then be extracted when listing out the posts, like an excerpt.